Power converters such as switch-mode voltage regulators are widely used in various electronic devices for sourcing power to the electronic devices from a power source.
Normally, a switching power converter may comprise a control circuit for controlling a switch module to switch on and off so as to convert an input voltage into an appropriate output voltage. When a power converter is switched off, an off process is needed so that the output voltage can be regulated to gradually decrease from a normal operating steady state value to a ground potential through controlling the on and off switching of its switch module. For example, for a buck type switching converter, its switch module typically comprises a high side switch and a low side switch connected in series between an input voltage and the ground potential. The control circuit of the buck type switching converter generally comprises a zero-crossing detection circuit which is used to prevent a low side switching current flowing through the low side switch from falling negative during normal operation. When the buck type switching converter is switched off, the zero-crossing detection circuit should be disabled to permit the low side switching current to be negative during the off process. However, should the buck type switching converter is switched off when it is operating in no load condition, once the low side switching current falls to negative during the off process, the buck type switching converter turns to a boost type switching converter. This is because the negative low side switching current will charge an input capacitor of the buck type switching converter connected to an input terminal of the converter. If the converter has a large output capacitor, such as an output capacitor of 1000 μF, in comparison of a relatively small input capacitor, such as of 20 μF, the input capacitor will be charged cycle by cycle, resulting in the input voltage increasing. In such circumstance, the input voltage may be increased until it exceeds an allowable safe operating voltage and damages the converter, leading to failure in switching off the buck type switching converter.